How many cubs can a mum raise? Maternal age and size influence litter size in polar bears

Life history theory predicts that females’ age and size affect the level of maternal investment in current reproduction, balanced against future reproductive effort, maintenance and survival. Using long-term (30 years) individual data on 231 female polar bears ( Ursus maritimus ), we assessed age- and size-specific variation on litter size. Litter size varied with maternal age, younger females had higher chances of losing a cub during their first months of life. Results suggest an improvement of breeding abilities early in life due to experience with subsequent reproductive senescence. Litter size increased with maternal size, indicating that size may reflect individual quality. Heterogeneity was observed among the largest females, suggesting that large size comes at a cost. Maternal traits and environmental conditions may act together to influence reproductive success.